Wendelstein 7-AS (1988 - 2002)

The Wendelstein 7-AS stellarator experiment, which was operated at IPP in Garching from 1988 till 2002, was the first test of basic elements of the stellarator optimisation. This first "Advanced Stellarator" thus paved the way for it's successor, Wendelstein 7-X in Greifswald.

The Wendelstein 7-AS stellarator experiment, which was operated at IPP in Garching from 1988 till 2002, belonged to the further developed generation of "Advanced Stellarators": Wendelstein 7-AS was distinguished from conventional stellarators by its re-computed, physically improved magnetic field generated by likewise innovative, specially shaped coils.

Technical data:

Major plasma radius

2 metres

Minor plasma radius

0,2 metre

Magnetic field

2,5 - 3,5 tesla

Number of coils

45

Plasma volume

1 cubic metre

Plasma quantity

< 1 milligram

Pulse length

5 seconds

Plasma heating

5,6 megawatts

Plasma temperature

15 - 60 million degrees

Objectives:

investigation of a net-current-free plasma

demonstration of improved equilibrium and transport behaviour due to the improved magnetic field structure

The experiment has demonstrated that the innovative modular coil system can be manufactured to the exact specifications. With plasma temperatures of 70 million degrees for the electrons and 16 million degrees for the ions, energy confinement times of up to 60 milliseconds, and reactor-grade plasma densities, Wendelstein 7-AS has broken all stellarator records in its size group.

The optimisation criteria used have also been confirmed: The troublesome displacement of the plasma column in the vessel as the plasma pressure increases – as compared with a conventional stellarator – is appreciably reduced in Wendelstein 7-AS. Its successor now being built at the Greifswald Branch Institute of IPP, the completely optimised Wendelstein 7-X device, is intended to demonstrate the reactor relevance of the new stellarators.

Wendelstein 7-AS was the first stellarator to be equipped with a divertor. Whereas previous devices restricted the outward extent of the plasma column by means of material limiters, Wendelstein 7-AS has done this contact-free since 1999 by means of magnetic fields: The plasma boundary splits – in keeping with the symmetry of the magnetic field – into individual offshoots through which energy and particles move to limited areas of the vessel wall, just like the divertor plasma in a tokamak. These areas of the wall are protected by special collector plates – ten such along the plasma column. Here the incident particles, together with the undesirable impurities from the plasma, can be neutralised and pumped off. This greatly facilitated impurity and density control; the plasma power could be sparingly distributed on the collector plates by radiation. The device thus provided advance information for Wendelstein 7-X, which will likewise be fitted with a divertor.

On July 31, 2002 Wendelstein 7-AS was put out of service to provide personnel and resources for its successor, Wendelstein 7-X in Greifswald.

In view of the world-wide stellarator program and in particular in preparation of Wendelstein 7-X the department Stellarator Heating and Optimization was responsible for the final documentation and evaluation of the results from Wendelstein 7-AS. A final review including extensive references: Hirsch et al., Plasma Phys. Control. Fusion 50 (2008) 053001. Further investigations were related to special topics such as the so called High-Density H-Mode Regime and the compilation of the Wendelstein 7-AS results into the International Stellarator/Heliotron Data Base.